The articles that were written for revolvers, apply to rifles as well - principle is the same.
In a revolver, the throats are the areas in each cylinder chamber immediately ahead of the portion of the chamber where the brass case rests and into which the bullet projects. If the bullet is sized so that it is a gentle force fit in the throat, all else being equal, your accuracy potential will increase greatly.
Two factors come into play here to improve potential accuracy:
a) the bullet axis is more nearly coaxial with the barrel axis.
b) gas cutting, which will positively RUIN accuracy, is virtually eliminated.
Gas cutting is the jetting of hot propellant gases past the bullet, causing erosion of the outer periphery of the bullet and plating some of that eroded lead onto the bore surface where the bullet runs over it, further increasing bullet degradation. When the throat is larger than the diameter of the bullet, gas cutting damage is severe even before the bullet hits the forcing cone, making good accuracy improbable. Even in rifles, where bullet diameter is more than about .0005" under throat size, gas cutting generally occurs along side the lands and disturbs bullet inte- grity.
The above is true of cast (non-jacketed) bullets more so than jacketed because they are far more subject to gas cutting. However, it would be a mistake to fail to realize the importance of a), above, for even jacketed bullets.
Despite what the gun writers say, my tests show that there are no observable pressure increases when using bullets up to .006" over barrel diameter. And it DOES NOT result in increased leading if other loading practices are reasonable.
Using an old C-H Swage-O-Matic bullet swaging press and appro- priately modified (opened) dies I am able to reshape and resize most bullets, including most of my cast bullet rejects, to usable and usually very accurate projectiles. If one does not require that the increase in diameter exceed a couple of thousands, and the bullet is not too hard, (no jacketed bullets here) a bullet sizer with the appropriate die can sometimes be use to "bump up" a given bullet to proper size altho this does not provide the same control and consistency as bullet swaging equipment. Be careful tho, it's easy to break the sizer handle.
Book after book has been written about revolver/cast/swaged bullet accuracy, some of them containing some highly misleading material. For anyone seriously interested in revolver accuracy, The Fouling Shot (now called The Cast Bullet), published every other month by the Cast Bullet Association is INVALUABLE! These are people who DO it, not just talk about it. I have improved a Redhawk .44 Magnum from a "best" of 2" at 25 yds. to a constant .75" at the same distance by changing bullet diameter alone. Now THAT'S satisfying!!!
I swage bullets (both cast and jacketed) to fit my revolver's cylinder throats snugly. As the Cast Bullet Association members have discovered, fitting the bullets to the gun's throats is probably the single most important step that one can take to improve a revolver's accuracy. NOTE: Elmer Keith was wrong! This is particularly true for lead bullets because it virtually eliminates gas cutting, but often helps with jacketed bullets also, mostly because of a), above.
I relate the above because my revolver's throats are oversize, as are most of the revolvers of all makes that I have owned in the past few years.
A more common and practical approach to obtaining a properly fit bullet is to have a mold cut for your throat and bullet alloy. Veral Smith of Cast Bullets Technology, does this work for little more than the price of a production mould. Ask him to cut the mould to cast bullets about .0005" under your throat size. Minor bullet diameter adjustments can be made by adjusting alloy cont- ent.
First thing I do with a new handgun is shoot it with some of my target loads (if appropriate) and if it does well I accept the gun the way it is. For reasons that no one seems to understand, some guns work well without "ideal" specs.
Until you have become a proficient shooter you will not see the difference anyway. As you improve, some of your guns will become unsatisfactory to you. However, before this happens you will probably change your likes and dislikes a dozen times for reasons other than accuracy. You will trade, sell, purchase and otherwise change your inventory quite extensively over the next few years.
Every shooter that I know who is a regular has done that; it's fun and is the richest learning period of one's shooting avoca- tion. For the time being, unless there is a serious problem with a given gun, just shoot lots to practice the fundamentals.
To obtain the diameter of a bullet that will shoot well in your revolver and will not require fancy equipment and techniques, do the following:
Take a SOFT lead bullet and squeeze it gently in a vice until it is about .010" oversize (say about .365" for a .38 cal. car- tridge). Attempt to push this slug through each throat of your cylinder using hand pressure only. If it will not pass through, your slug is large enough. If it will pass you must increase the diameter slightly.
Before doing the following, remove the cylinder from the gun, clean it of ALL lead or copper fouling, lube it lightly and make sure that it is solidly supported on an appropriate wooden block.
With an appropriately sized brass drift, carefully drive the slug through each throat from the end of the chamber where the cartridge is normally inserted. What you are attempting here is to find the diameter of the smallest throat, so handle the soft lead carefully and drive it straight to avoid distorting the slug.
This may take a couple of practice runs because the transition between the area of the chamber where the brass case resides and the area into which the bullet projects (throat) varies from cylinder to cylinder. Some cylinders have a sharp step while others have a relatively long smooth transition. If the gun that you are working with has a sharp step it may be easier to mini- mize the distortion by driving it from the other end of the cylinder.
After you have completed the above, you have the "ideal" bullet diameter to be fired in your gun and you will be very fortunate if the bullet is the nominal diameter for that caliber. Usually you will find that it is somewhat larger. For the time being don't worry about the difference in the throats in a particular cylinder. I find that even though they are sometimes badly oversize, they usually do not vary much more than about .0005".
An exception that would negate this approach would be if you encountered a barrel that slugs larger than your throats. The only solution to this unfortunate situation is to either open the throats to .001" or .002" larger than the bore or to get a new gun. Fortunately, this occurrence is rare (I haven't yet run into the problem, but have read of others who have).
Another exception is the condition where the oversize bullet is so large that when it is loaded into the brass, the cartridge cannot be chambered. In cases where this condition is just marginal, a taper crimp will solve the problem. Where it is more extreme, the chamber (but NOT throat) will need to be enlarged to accommodate the cartridge, although if the gun is new, return it to the manufacturer. The only gun in which I have had this experience was a stainless Virginian Dragoon. Interarms sent me a new cylinder to solve the problem.
USE ONLY ENOUGH TAPER CRIMP TO HOLD THE BULLET SECURELY! Other- wise you will size the bullet as it enters the case and ruin all of these grand efforts.
Remember that when shooting cast bullets, the size of the expander plug is very important, and if one uses a plug of the standard size normally supplied by the manufacturer, the oversize bullet may be sized down when it is seated in the brass case thus ruining all of the above effort. Your expander plug for cast bullets should be .001 to .002 smaller than your new bullet diameter.
The way that I accomplish this is to keep a supply of next- caliber-up Lyman expander plugs, then when I need a particular size, I con a machinist friend to turn one to the proper diameter. The Lyman is an excellent design because it applies a step at the case mouth that makes for an easy way to start the bullet into the case with the bullet axis concentric with the case axis, a very important characteristic for pistol and rifle ammunition.
Still another problem, although not a cylinder problem, is one that is directly related, but that few shooters know exists. In revolvers, the area of the bore directly adjacent to the forcing cone where the barrel is screwed into the frame is sometimes smaller than the rest of the bore. This damages the bullet the same as does an undersized throat and, unfortunately, is a very common occurrence.
The solution is to lap the bore with a cast bore lap to remove the constriction. That is not the easiest job for the inexperi- enced. An excellent alternative is to shoot several hundred rounds (200-500) depending on the amount of constriction, charged with a proper lapping compound. This has the added advantages of polishing the rest of the bore and providing a very slight taper to the bore for excellent cast bullet shooting and will do a great job of minimizing leading. It is important that a PROPER lapping compound be used. Clover's Valve Grinding Compound is much too coarse. Veral Smith of Cast Bullets Technology has the proper stuff for about $10.00.
Another accuracy tipWhen shooting the longer, fitted bullets, load them as far out into the throat as is practically possible. This assures true concentricity of bullet and bore axes for precise launch into the bore. It makes the cartridge fit independent of a sloppy chamber because the bullet supports the case rather than the reverse. I load my heavy bullet .45 Colt, .44 Magnum, and .357 loads to clear the front of the chamber by about only .10". Of course, the short bullets do not allow seating out very far.